1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device having a direct-type backlight which is suitable for improving uniformity of luminance.
2. Discussion of the Related Art
Recently, various flat display devices have been developed, for example, a Liquid Crystal Display (LCD) device, a Plasma Display Panel (PDP), an Electro-Luminescent Display (ELD) and a Vacuum Fluorescent Display (VFD). Among the various flat display devices, the LCD device has been most widely used due to advantageous characteristics of thinness, lightness in weight, and low power consumption. In addition to use as a mobile-type LCD device such as a display for a notebook computer, the LCD device has been developed as a display device for use with computers, television receivers, and for similar applications.
The LCD device uses the optical anisotropy and polarization characteristics of liquid crystal. A liquid crystal is comprised of liquid crystal molecules having long and thin structures, whereby the liquid crystal has electric-field-related alignment characteristics. That is, it is possible to control the orientation of the liquid crystal molecules by applying an electric field to the liquid crystal. Accordingly, when controlling the alignment direction of the liquid crystal molecules, the properties of the liquid crystal are changed, whereby light is refracted in the alignment direction of the liquid crystal by the optical anisotropy, resulting in the display of an image or other information.
Generally, the LCD device includes an LCD panel having a liquid crystal layer between two substrates, a driving circuit for driving the LCD panel, and a backlight for providing a light source to illuminate the LCD panel from the side opposite the viewer.
In the LCD panel, a lower substrate having a thin film transistor (TFT) array thereon is bonded at predetermined intervals to an upper substrate having a color filter array thereon, and then the liquid crystal layer is formed between the lower and upper substrates, whereby a image is displayed by controlling a transmitted light according to a voltage applied to each pixel of the TFT array.
The LCD panel does itself not emit light, so that the LCD panel uses an ambient light source, or requires an additional light source. Accordingly, the LCD panel may be classified into a reflective-type LCD panel and a transmitting-type LCD panel according to the type of the light source. That is, the transmitting-type LCD panel uses an additional light source, and the reflective-type LCD panel uses an ambient light source. A transflective-type LCD panel uses both the additional light source and the ambient light source. Herein, the light source for providing uniform light to the transmitting-type or the transreflective-type LCD panel is referred to as a backlight.
As the backlight consumes electrical power, it is desirable to emit the brightest light with a minimum power dissipated in the backlight. The backlight may be classified into a direct-type and an edge type according to a position of a fluorescent lamp. In the direct-type, the fluorescent lamp emits the light to the entire rear side of the LCD panel. Meanwhile, in the edge-type, the fluorescent lamp is located at an edge of the LCD panel, and the light emitted from the fluorescent lamp is directed to the rear side of the LCD panel through a light-guiding plate.
In the direct-type backlight, a plurality of fluorescent lamps may be formed below the LCD panel, or a bent fluorescent lamp may be formed. A predetermined separation between the fluorescent lamps and the LCD panel must be maintained to prevent the silhouette of the fluorescent lamps from being observed on the LCD panel. Also, a light-scattering means is provided to obtain uniform light intensity. Thus, it is hard to obtain a thin LCD device when using the direct-type backlight therein. In the edge-type backlight, a light-guiding plate is used for uniformly scattering the light to an entire surface of the LCD panel, and it has problems in that the luminance is low. Accordingly, the edge-type backlight is used in an LCD device requiring thinness, e.g., a notebook computer, and the direct-type backlight is used in the LCD device requiring large sized screen and high luminance.
Hereinafter, a related art LCD device having a direct-type backlight will be described with reference to the accompanying drawings. FIG. 1 is a cross-sectional perspective view illustrating an LCD device having a related art direct-type backlight, and FIG. 2 is a perspective exploded view illustrating a related art direct-type backlight.
As shown in FIG. 1, the related art LCD device includes an LCD panel 1, a plurality of fluorescent lamps 5, optical sheet 2 and diffusion plate 3, a reflecting plate 4 and a case 8. The LCD panel displays a picture image by controlling a transmitted light, and the plurality of fluorescent lamps 5 are formed below the LCD panel at fixed intervals. Also, the optical sheet 2 and diffusion plate 3 are formed at a rear surface of the LCD panel 1 to prevent the silhouette of the fluorescent lamps 5 from being observed at a display surface of the LCD panel 1, and to provide a light source having uniform luminance. A reflecting plate 4 is formed below the fluorescent lamps 5 to reflect the light emitted from the fluorescent lamps 5 to the LCD panel 1, and the case 8 supports the fluorescent lamps 5 and the reflecting plate 4.
An air layer is formed between the plurality of fluorescent lamps 5 and the optical sheet 2 and diffusion plate 3, and an air layer is not formed between the LCD panel 1 and the optical sheet 2 and diffusion plate 3.
Hereinafter, the direct-type backlight will be described in detail.
As shown in FIG. 2, the direct-type backlight includes the plurality of fluorescent lamps 5 formed at fixed intervals, the case 8 fixing and supporting the fluorescent lamps 5, the optical sheet 2 and diffusion plate 3 formed between the LCD panel 1 and the fluorescent lamps 5, and the reflecting plate 4 formed on an inner side of the case 8 to reflect the light emitted from the fluorescent lamps 5 on a display part of the LCD panel.
The reflecting plate 4 and the case 8 are formed of a material having high heat conductivity such as the aluminum group. Also, the fluorescent lamps 5 are Cold Cathode Fluorescent Lamps (CCFL), in which electrodes are formed at both ends thereof. That is, when a power is applied to the electrodes at both ends of the lamp, the lamp emits the light. Also, the both ends of the fluorescent lamp 5 are inserted into holes formed at a side of the case 8.
In the direct-type backlight of the related art LCD device having the aforementioned structure, the light passage is as follows: when the power is applied to the fluorescent lamp 5, the fluorescent lamp 5 emits light; part of the emitted light is directly incident on the diffusion plate 3 and the optical sheet 2, and some is reflected by the reflecting plate 4, and then is incident on the diffusion plate 3 and the optical sheet 2; the diffusion plate 3 scatters the incident light, so that the light is uniformly incident on the optical sheet 2; the optical sheet 2 concentrates the incident light through the diffusion plate 3, thereby improving luminance. Subsequently, the light is incident on the LCD panel 1, whereby the image is displayed.
If the fluorescent lamps 5 are formed too close to the LCD panel 1, a silhouette of the fluorescent lamp 5 may be observed on the LCD panel. Accordingly, in order to prevent a silhouette of the fluorescent lamp 5 on the LCD panel, it is desirable to maintain a predetermined interval between the fluorescent lamp 5 and the diffusion plate 3, whereby an open area is formed between the fluorescent lamp 5 and the diffusion plate 3.
Also, when the light emitted from the fluorescent lamp 5 is incident on the LCD panel, the fluorescent lamp 5 emits heat in addition to light. Part of the heat emitted from the fluorescent lamp 5 is directly transmitted to the diffusion plate 3 and the optical sheet 2, and the heat transmitted to the diffusion plate 3 and the optical sheet 2 is transmitted to the LCD panel 1. Some of the heat is absorbed by the reflecting plate 4, and is transmitted to the case 8, and then is emitted to the environment external to the LCD device.
The related art direct-type backlight has the following disadvantages.
As a size of the LCD panel increases, the number of the fluorescent lamps 5, which are required in the direct-type backlight increases, whereby the heat generated in the fluorescent lamps 5 increases.
The heat generated by the fluorescent lamps is emitted through the surface of the case, so that the emitted heat is relatively small. Thus, the internal temperature of the backlight increases (to about 80° C.). This shortens the lifetime of the fluorescent lamp, and the heat generated from the backlight has detrimental effects on the LCD panel, thereby causing problems in operating the LCD panel.
In order to control the increase of the internal temperature in the backlight, it is possible to use a cooling fan. But, the unit manufacturing cost of the LCD device increases, and the power consumption increases. Also, the cooling fan generates noise.